The present invention relates to a method for the uniform coating of a honeycomb body formed of ceramic or metal with an amount of the solid matter of a coating dispersion which amount, at a given constant density of the coating dispersion, is below that which arises under equilibrium conditions between the honeycomb body and the coating dispersion.
Up to the present invention, when coating porous ceramic carriers of the monolithic type with finely divided oxides, e.g. catalytically active Al.sub.2 O.sub.3 for increasing the surface of a subsequently applied catalytically active component, the carriers were immersed in an aqueous dispersion of the oxide until the complete filling up of the cells occurred or the carriers were flooded with the dispersion. Then, the excess dispersion remaining in the cells was conventionally removed by blowing it out or by removing it by suction (Ullmanns Encyclopedia of Industrial Chemistry, 3d German edition, vol. 9 (1957), pp. 273, 274). Coatings were always created thereby in accordance with the complete saturation of the water-pore volume of the particular ceramic carrier. The known method only permits the use of relatively low-concentrated, that is, highly fluid oxide dispersions, so that multiple coatings with intermediate drying is required in order to apply an amount of oxide which is suitable for the intended purpose The amount of oxide absorbed in the individual instance per carrier is strictly related to the porosity and the absorption capacity of the carrier, so that in many carriers the resulting amounts of oxide loading vary sharply. In addition, the immersion process must be carried out so slowly that no flooding of the front surface on top occurs during immersion and with resulting inclusion of air in the monolithic honeycomb body before the rising liquid column of the dispersion reaches this front surface. If this slow process is not used, uncoated areas remain in the monolithic honeycomb body.
A technically rather expensive method described in DE-AS 25 46 489 circumvents the problem of a simple immersion of a monolithic honeycomb body by means of introducing the honeycomb body into a pressure chamber, evacuating this chamber and the honeycomb body located in it including its pores, flooding the chamber with the dispersion and building up an excess pressure in the chamber in order to force the dispersion into the pores as well as subsequently removing the excess dispersion remaining in the cells of the monolithic honeycomb body. This publication does not contain a teaching for a uniform application of desired amounts of solid matter.
An object of the invention is to overcome these disadvantages and create a coating method with which a desired amount of coating can be applied in a uniform distribution, independent of a given absorption capacity of a porous ceramic carrier or of a given adsorption action of a metallic carrier, and the scattering of the charged amounts of the coating within a carrier series can be reduced.